1. Field of the Invention
The present invention relates to a conductive resin composition. More particularly, the present invention is concerned with a conductive resin composition produced by melt-kneading a mixture comprising a polyamide (A), a polyphenylene ether (B), a rubbery polymer (C), a conductive carbonaceous material (D), and a low molecular weight modifier compound (E) which functions as a compatibility agent for the polyamide (A) and the polyphenylene ether (B), wherein the amounts of the conductive carbonaceous material (D) and the low molecular weight modifier compound (E) which are contained in the mixture are within respective specific ranges, and the amount of a volatile substance contained in the polyphenylene ether (B) is also within a specific range. The conductive resin composition of the present invention not only exhibits excellent impact strength and heat stability, but also is capable of suppressing die build-up (i.e., deposit of a resin at a die outlet of an extruder which increases with the lapse of time during the extrusion) and strand breakage which occur during the extrusion molding thereof. Such a conductive resin composition can be used in a wide variety of fields, such as electric and electronic parts, parts of office automation machines, automobile parts and other mechanical parts. Especially, even when used as a raw material for producing a large shaped article, the resin composition exhibits excellent heat stability during the residence thereof in a molding machine, and the resultant shaped article has not only conductivity sufficient for electrostatic coating but also excellent coating adhesion strength even after exposure of the shaped article to heat. Therefore, the conductive resin composition of the present invention is very advantageous as a raw material for producing a large shaped article, such as an automobile outer panel, which is subjected to an electrostatic coating.
2. Prior Art
Polyphenylene ethers not only have excellent mechanical and electrical properties and excellent heat resistance, but also have excellent dimensional stability. Therefore, polyphenylene ethers have been used in a wide variety of fields. However, the moldability of a polyphenylene ether is poor. For improving the moldability of a polyphenylene ether, Examined Japanese Patent Publication No. Sho 45-997 (corresponding to U.S. Pat. No. 3,379,792) discloses a technique in which a polyamide is added to a polyphenylene ether, to thereby obtain a polyamide-polyphenylene ether alloy. Nowadays, polyamide-polyether alloys are used in a very wide variety of fields.
Recently, as an electrostatically coatable material, use of a conductivity-imparted polyamide-polyphenylene ether alloy is rapidly expanding in the fields of automobile outer panels (such as a fender and a door panel). For example, it has been attempted to use an automobile fender made of a polyamide-polyphenylene ether alloy for the purpose of improving not only the safety of automobiles (e.g., protection of pedestrians) but also the ability of an automobile fender to recover from distortion.
The materials used for an automobile outer panel are required to have various good properties, such as a conductivity sufficient for electrostatic coating, impact resistance, heat resistance and melt-fluidity.
With respect to the method for imparting a conductivity to a polyamide-polyphenylene ether alloy, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-201811 discloses a technique to lower the surface resistance of a polyamide-polyphenylene ether alloy by a method in which carbon black is caused to be contained mainly in the polyamide phase of the polyamide-polyphenylene ether alloy, or by a method in which carbon black is uniformly dispersed in a polyamide, followed by mixing thereof with a polyphenylene ether.
Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-48869 (corresponding to U.S. Pat. No. 5,977,240) discloses a technique in which a polyamide and a polyphenylene ether are compatibilized with each other in advance, followed by addition of carbon black, to thereby obtain a resin composition having good impact strength, good melt-fluidity, and excellent (low) volume resistivity.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 4-300956 (corresponding to EP 506386) describes that, in the production of a composition containing a polyamide, a polyphenylene ether and carbon black, by adjusting the amounts of these components and using a polyphenylene ether and a polyamide which have specific relative viscosities, it becomes possible to improve the conductivity and processability of the composition.
International Patent Application Publication No. WO 01/81473 discloses a technique in which a conductive carbonaceous filler (Ketjen Black) is caused to be present in a particulate form in the polyphenylene ether phase of a polyamide-polyphenylene ether resin composition. Further, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. 2002-544308 (corresponding to U.S. Pat. No. 6,221,283) discloses a method for producing a resin composition containing a dispersed phase polymer, a continuous phase polymer, a conductivity imparting agent and a compatibility agent in which the amount of the compatibility agent is adjusted to control the particle size of the dispersed phase polymer in the continuous phase polymer, to thereby achieve a desired volume resistivity of the resin composition. In addition, Unexamined Japanese Patent Application Laid-Open Specification No. 2001-302904 discloses a technique in which a modified polyphenylene ether obtained by modifying a solid polyphenylene ether is melt-kneaded with a polyamide and an inorganic filler (such as carbon black), to thereby obtain a resin composition having excellent appearance, impact resistance and heat resistance.
However, due to the addition of a conductive filler, each of the above-mentioned techniques poses a problem in that, when the production of a resin composition is performed using a large extruder for a long time, die build-up (i.e., deposit of a resin at a die outlet of an extruder which increases with the lapse of time during the extrusion) is likely to occur. Further, when the obtained resin composition is shaped into a large article (such as an automobile outer panel), disadvantages are caused in that the heat stability during the residence of the resin composition in a molding machine is lowered, the molding conditions are restricted, and the productivity of shaped articles is markedly lowered.
In the fields where a shaped article needs to be subjected to an electrostatic coating, it is important that the shaped article has excellent appearance so as to ensure a satisfactory level of coating adhesion strength and sharpness of an image reflected on the coated shaped article. In the field of automobile fenders, sometimes after an online electrostatic coating of a shaped article, the coated shaped article is subjected to a heat treatment together with a metal panel so as to cure a rust resisting paint coated on the metal panel. Such a heat treatment is generally performed at about 170° C. to 200° C. or at a higher temperature for 10 to 50 minutes. Therefore, the resin composition used in the field of automobile fenders is required to have, in particular, excellent coating adhesion strength not only just after the molding process but also after heat exposure, as well as excellent heat resistance and heat stiffness.
However, conventionally, it has been impossible to produce a resin composition having properties required of a raw material used for producing an automobile fender, i.e., a resin composition which not only has excellent balance of impact resistance, stiffness and conductivity, but also is improved with respect to the heat stability during the residence in a molding machine and the coating adhesion strength after heat exposure. Therefore, the development of a new technique to produce such a resin composition has been desired in the art.